1. Field of the Invention
The invention is generally related to Global Positioning System (GPS) receivers, and in particular, to partial processing of a GPS signal for relaying to a remote processing station.
2. Description of the Related Art
The Global Positioning System (GPS) Operational Constellation nominally comprises 24 earth orbiting satellites. Each satellite radiates a spread spectrum, pseudorandom noise (PN) signal indicating the satellite's position and time. A GPS receiver tuned to receive the signals from the satellites can compute the distance to the satellites and calculate the receiver's position, velocity, and time. The receiver calculates the distance to a satellite by multiplying the propagation rate of the satellite's radio signal, i.e., the speed of light, by the time it took the signal to travel from the satellite to the receiver.
Each satellite transmits two carrier signals referred to as L1 and L2. L1 operates at a frequency of 1.57542 GHz and L2 operates at a frequency of 1.22760 GHz. Multiple binary codes induce phase modulation upon the L1 and L2 carrier signals. Each satellite in the GPS Operational Constellation transmits a unique code over the L1 and L2 carrier signals. One of the phase-modulated signals is C/A Code (Coarse Acquisition Code). Presently, 32 codes are defined for the C/A Code. A satellite's C/A Code phase modulates the L1 carrier over a 1.023 MHz bandwidth. The C/A Code is a repeating 1023 bit sequence. At 1023 bits and 1.023 MHz, the C/A Code repeats every millisecond. The C/A Code forms the basis for the Standard Positioning Service (SPS) used by civilians.
Another phase-modulated signal is the P-Code (Precise Code). The P-Code is similar to the C/A Code in that it is a PN sequence which phase modulates a carrier signal. The P-Code modulates both the L1 and the L2 signals at a rate of 10.23 MHz. In an Anti-Spoofing mode, the P-Code is encrypted to produce the Y-Code to restrict access to users with the encryption key. The P-Code forms the basis for the military's Precise Positioning Service (PPS). It will be understood that additional signals can be added to existing carriers or to additional carriers.
One use of GPS is tracking of a moving vehicle. A GPS receiver is mounted in a vehicle, and the GPS receiver sends information to a remotely located base station. Such information can be used for testing and evaluation of the vehicle's performance, for precise information regarding the location of the vehicle at a particular time, and the like. Some vehicles present high-dynamic environments to the GPS receiver and are difficult to track with precision. Examples of high-dynamic environments with large amounts of acceleration (g-forces) and jerk (rate of change of acceleration) include reentry vehicles, high-performance aircraft, racing cars, and the like. Unmanned vehicles, such as interceptor missiles and munitions, can feature even higher dynamics than manned vehicles. For example, an interceptor missile can exhibit acceleration rates of up to 75 g's and jerk rates of up to 75 g's per second.
Knowledge of the location of a high-performance vehicle is instrumental for safety. For example, if a prototype unmanned aircraft is being tested at supersonic speeds, it may be desirable for testing personnel to possess accurate real-time information about the test plane's position and velocity. Should the tracking provided by the GPS receiver indicate that the test plane is out of control or is lost, a range safety officer may decide to destroy the test plane due to safety concerns. An inaccurate or unreliable GPS receiver may also evoke a similar response from the range safety officer, which can undesirably result in the needless waste of expensive prototypes and loss of development time. Other high-dynamic environment tracking applications include downrange tracking, midcourse platform correction, and miss-distance calculations.
Standard, off-the-shelf GPS receivers do not perform well in high-dynamic environments. Due to the relatively great distance between a GPS receiver and a GPS satellite, the received GPS signal strength is low, and a carrier phase locked loop (PLL) in a GPS receiver typically has relatively narrow bandwidth. Relatively high dynamics induce tracking errors in these narrow-band carrier phase locked loops. The component of vehicle dynamics that stresses a carrier loop is the projection of the vehicle motion vector onto the satellite ray path to the vehicle. When these loop tracking errors are large enough, carrier cycles may be missed, which can lead to errors in position tracking. Under extreme conditions, the tracking errors can result in a complete loss of lock. Furthermore, the satellite signals that are least affected by dynamic events and are likely to remain in lock are also those that would provide the least information about tracking through the dynamic event, e.g., a signal from a satellite that is perpendicular to the direction of movement is least affected. These tracking errors render it impractical to use a standard unaided GPS receiver in an environment with more than about 12 g's of acceleration.
To cope with high-dynamic environments, a conventional GPS receiver may be augmented with an inertial measurement unit (IMU). The measurements of vehicle acceleration from the IMU are projected onto the various satellite ray-paths and used to steer the corresponding satellite carrier tracking loop in the receiver to prevent tracking errors. Disadvantageously, such IMUs are large, expensive, and heavy.
GPS translation is another conventional technique that can be used in these high-dynamic environments. With GPS translation, a GPS receiver is apportioned to a GPS translator (front end) and a back end. The translator or front end of the GPS receiver is onboard the vehicle to be tracked. The back end can be conveniently located remotely, such as at a monitoring station on the ground, and the monitoring station can more easily accommodate advanced processing that is not practical to implement within the space and power confines of an on-board processor. A conventional translator system receives a signal and repeats the data received. To prevent interference between the received signal and the transmitted signal, the carrier frequency of the transmitted signal can be different than the carrier signal of the originally received signal. In a conventional translator system for GPS, a selected carrier channel, such as the L1 channel for C/A codes, is received by the translator and transmitted to the back end. It will be understood that even though the GPS satellites broadcast GPS signals at the same frequencies, the received frequencies from various satellites may vary due to Doppler shift. As a result, relatively many satellites may be tracked, and relatively many carriers may be translated. Disadvantageously, conventional translation techniques occupy relatively large amounts of bandwidth, thereby rendering conventional translation techniques impractical.